Engine idle speed control is typically accomplished in stoichiometric engines by adjusting airflow to control engine speed to a desired engine speed. However, since airflow control may be slow due to manifold volume dynamics, ignition timing adjustment is also used. To allow for ignition timing adjustment to increase engine torque, nominal operation of idle speed control requires some basic amount of ignition timing retard. This basic amount of ignition timing retard, which is substantially present during all idle speed control, produces a negative impact on vehicle fuel economy since optimum torque (optimum ignition timing) is not always employed. Further, the range of authority using ignition timing alone is limited.
Another approach for controlling idle speed uses a lean burn engine. In this approach, fuel can be adjusted to quickly control engine idle speed. Here, ignition timing retard during normal engine auto speed control is not required to the extent as in stoichiometric engines, since adjustment of fuel injected can quickly change engine torque.
The inventors herein have recognized disadvantages with the above approach. In particular, since idle speed operation requires very low engine torque (just enough to cancel friction and power accessories), the engine cylinders operate at a low torque level. Thus, when operating lean at these low torque levels, engine combustion stability is degraded. This degraded combustion stability can produce poor customer perception, and degraded speed control.